I'm not familiar with the Langelier Index. If you are working in a fairly constant medium, then it probably would work. If pH is low because of iron precipitation, you are unlikely to precipitate carbonates. If pH is neutral or higher, pH is probably the most important quantity. The saturation index depeneds on Ca, Alkalinity, and pH. pH is a log quantity while the saturation index will depend on the log of Ca and alkalinity, which will only change .3 units if for example the Ca concentration doubles. However, if you have the data, you might as well do the speciation and calculate the saturation index. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled Richard Kane <kane@xxxxxxxxxx To: 'David L Parkhurst' <dlpark@xxxxxxxx> m> cc: In-Reply-To: <41EF01775B43D211B29800A0C9E0C0FF363734@xxxxxx> Subject: Langelier Sat Index 07/30/02 02:40 PM David, Is the Langelier Index (LSI = pH - pHs) too simplistic for determining calcium carbonate precipitation, when the water system also contains iron precipitation which will tend to drive down pH, thereby using available alkalinity for buffering (and not calcium precipitation)? Tad Kane Staff Scientist The marvels of modern technology include the development of a soda can which, when discarded, will last forever, and a $7,000 car which, when properly cared for, will rust out in two or three years. -Paul Harwitz email: kane@xxxxxxxxxxx XDD,llc 16 MARIN WAY, STRATHAM, NH 03885 603-778-1100 603-778-2121 fax -----Original Message----- From: David L Parkhurst [mailto:dlpark@xxxxxxxx] Sent: Tuesday, July 02, 2002 12:27 PM To: Richard Kane In-Reply-To: <41EF01775B43D211B29800A0C9E0C0FF363734@xxxxxx> Subject: RE: Phreeqc simulations > I've run a number of simulations using exact monitoring point data for the major cations, along with alkalinity and sulfate (the only significant anion data I have - nitrate, phosphate are miniscule). Regardless of the data set, I'm always short in the anions department for the charge balance. You need to get a better analysis. > So, I decided to use total carbon as C(4) with a charge balance instead of alkalinity for an input, because I can think of no other way of balancing the solution. I converted alkalinity as CaCO3 (430 ppm) to HCO3- (525 ppm) (assuming the 2eq/mole ratio thereby getting a higher concentration of HCO3-), and added it to the field measured CO2 (560 ppm) to get an approximate total dissolved carbonate species value of 1085 ppm. I used this value for C(4). This assumption is driving the results of the calculation. If there is not as much alkalinity as you are defining, no calcite will precipitate at all. The iron will oxidize and generate low pH water that is undersaturated with calcite. My guess is that the alkalinity is not high enough to buffer the pH if there are large iron conentrations. With these high iron waters, alkalinity should be done in the field, by precipitating iron on th way to the lab, the lab-measured alkalinity values would be erroneous. For landfill leachate, some of the missing anions may be organic acids, so a titration to a very low pH would be a good idea to see if there are any organic acids that are titrated in the pH 2-4 range. > The solution is then allowed to react with the specified mineral assemblage, which includes keeping CO2(g) and O2(g) near atmospheric. The model is predicting 3.032e-003 moles of calcite, 1.880e-003 moles of goethite, and 3.458e-004 moles of manganite will precipitate at equilibrium conditions (all have SI=0.0). The pH shifted from 6 to 8.8 and the pe from 0 to 11.8. Accordingly, with this shift, the dominant carbon species have changed from CO2 and HCO3- to HCO3- and CO3-2. Other minerals are oversat'd w.r.t their components in solution, but due to the fact that they are unreactive (slow formation kinetics), they are not expected to precipitate. > So, with that said, my major uncertainty remains. If my main concern is to see whether or not enough carbonate is present in the pH>7 / aerobic trench waters (modeled by designating the CO2(g) and O2(g) is the phase assemblage) to precipitate all the Ca2+, then is it inaccurate to have the charge balance on the C(4)? I think so. > In this way, it doesn't matter what initial C(4) you enter, 1 or 1,000,000, PHREEQ will always adjust this initial concentration upward, to come into balance with the cations. So, this elevated C(4) concentration is carried into EQUILIBRIUM PHASES, where it is allowed to react with the phase assemblage. If the model is always elevating the C(4), won't I always have all the calcium precipitating out? Yes, your assumption is producing the alkalinity that you need. Run the calculation with the original alkalinity (236?) and look at the results. The key is whether there really is enough buffering capacity to buffer the acid generated by iron oxidation and precipitation; you need better analytics or a backup (lime?) to make sure you don't generate more acid than can be buffered. > And if so, is this correct? I feel as though I'm not hitting the mark on what I'm trying to accomplish. I don't think you can answer your question without knowing the alkalinity and the missing anions with more certainty. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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